Embodiments relate generally to a power supply for image intensifiers, and more particularly to a clamped cathode power supply for an image intensifier that provides improved bright source resolution.
Image intensifiers are well known for their ability to enhance night-time vision. The image intensifier amplifies the incident light received by it to produce a signal that is bright enough for presentation to the eyes of a viewer. These devices, which are particularly useful for providing images from dark regions, have both industrial and military application. The U.S. military uses image intensifiers during night-time operations for viewing and aiming at targets that otherwise would not be visible. Low intensity visible spectrum radiation is reflected from a target, and the reflected energy is amplified by the image intensifier. As a result, the target is made visible without the use of additional light. Other examples include using image intensifiers for enhancing the night vision of aviators, for providing night vision to sufferers of retinitis pigmentosa (night blindness), and for photographing astronomical bodies.
FIG. 1 depicts an exemplary image intensifier 10. A typical image intensifier 10 includes an objective lens 12, which focuses visible and infrared radiation (collectively referred to herein as light) from a distant object onto a photocathode 14. The photocathode 14, a photoemissive semiconductor heterostructure that is extremely sensitive to low-radiation levels of light in the 580-900 nm spectral range, provides a spatially coherent emission of electrons in response to the electromagnetic radiation. Electrons emitted from the photocathode 14 are accelerated towards the input of the microchannel plate (MCP) 20. The MCP 20 amplifies the incident electrons in a spatially coherent manner. Electrons emerging from the output of the MCP 20 are accelerated toward the phosphor screen 16 (anode), which is maintained at a higher positive potential than the output of the MCP 20. The phosphor screen 16 converts the emitted electrons into visible light. An operator views the visible light image provided by the phosphor screen through an eyepiece 18.
Amplification of the ambient light incident on the image intensifier is achieved by placing an MCP 20 between the photocathode 14 and phosphor screen 16. The MCP 20 is a thin glass plate having an array of microscopic holes through it used to increase the density of the electron emission. Electrons impinging on interior sides of the holes through the MCP 20 result in the emission of a number of secondary electrons each of which, in turn, causes the emission of more secondary electrons. Thus, each microscopic hole acts as a channel-type secondary emission electron multiplier having a gain of up to ten thousand. The electron gain of the MCP 20 is controlled primarily by the potential difference between its input and output planes. A power source 22 applies power to the photocathode 14, the MCP 20 and the phosphor screen 16.
There are several methods used to extend the useful range of night vision intensifiers to and beyond light levels of approximately 10-1 footcandles. These methods modify the voltage of the cathode relative to the input of the MCP (cathode voltage) in response to input light levels.
The simplest method reduces the effective DC potential of the cathode. This is achieved by placing a high value resistor (Bright Source Protection, BSP) in series between the cathode DC power supply and the cathode. The voltage drop caused by cathode current flowing through the BSP reduces the cathode voltage and thereby reduces the accelerating potential between the cathode and the MCP. The reduced accelerating potential reduces the MCP input current somewhat.
In addition to modifying the DC potential of the cathode, some night vision intensifier power supplies impose AC signals on the cathode as well. The AC signals alternately drive the cathode into and out of conduction, reducing the MCP gain by reducing the effective input current at high light levels. Two types of AC modulation of the cathode are in common use. These are AC clamping and autogating.
Existing AC clamping of the cathode consists of superimposing a half wave rectified sinusoid on the cathode. The sinusoid is referenced to the MCP input and is coupled to the cathode of the intensifier through a diode. The anode of the diode is connected to the cathode of the intensifier. Under high light conditions the cathode of the intensifier is driven with a negative going half wave rectified sinusoid. The sinusoid is typically between 25V peak and 50V peak and 10 KHz and 50 KHz.
Autogating consists of superimposing a duty cycle modulated pulsed waveform on the cathode. The pulse waveform is referenced to the MCP input and is often capacitivly coupled to the intensifier cathode. The autogating waveform on the cathode alternately enables and disables cathode conduction. The autogating duty cycle is controlled by and responds to the light level. Increasing light levels actively reduce the conduction time of the image intensifier, thus reducing the input current and allowing use at higher light levels.
There are drawbacks to both methods of extending the useful range of night vision intensifiers. The existing AC clamping technique is straightforward to apply but extends the useful range by a minimal amount. Autogating does extend the useful range of image intensifiers by an order of magnitude or greater, but requires active electronics to sense the light level and modulate the duty cycle.
While there are methods to extend image intensifier performance in high light environments, further improvements in high light resolution of image intensifiers would be well received in the art